1. Field of the Invention
The invention relates to a communication station which is arranged for wireless communication with at least one transponder and which includes carrier signal generating means whereby a carrier signal having a given frequency can be generated, which carrier signal can be modulated by the transponder during the wireless communication with a transponder, thus forming a modulated carrier signal which has a sideband in the range of a sideband frequency. The communication station also includes a transmission coil configuration with an active, first transmission coil circuit which has a flat, first transmission coil wherethrough a coil current can be driven, with first capacitor means connected in parallel with the first transmission coil, and with a passive, second transmission coil circuit which has a flat, second transmission coil which is oriented so as to extend essentially parallel to the first transmission coil, is arranged opposite the first transmission coil and is inductively coupled to the first transmission coil. The inductive coupling to the first transmission coil results in a coil current which is shifted essentially 90xc2x0 relative to the coil current that can be driven through the first transmission coil to be driven through the second transmission coil. Second capacitor means are connected parallel with the second transmission coil, and has a resonant frequency in the range of the carrier signal frequency.
The invention also relates to a transmission coil assembly for use in a communication station for wireless communication with at least one transponder, and which is provided with an active, first transmission coil having a flat, first transmission coil wherethrough a coil current can be driven, first capacitor means connected in parallel with the first transmission coil, and a passive, second transmission coil circuit having a flat, second transmission coil which is oriented so as to extend essentially parallel to the first transmission coil. The second transmission opposite the first transmission coil and is inductively coupled there to the inductive coupling resulting in a coil current in the second transmission coil which has been phase shifted essentially 90xc2x0 relative to the coil current in the first transmission coil. Second capacitor means are connected in parallel with the second transmission coil, and has a resonant frequency in the range of the frequency of a carrier signal which can be modulated by the transponder during the wireless communication with a transponder, resulting in a modulated carrier signal which has a sideband in the range of a sideband frequency.
2. Description of Related Art
Various versions of a communication station of the kind described in the first paragraph and of a transmission coil configuration of the kind described in the second paragraph are already commercially available and used in practice.
In communication stations and transmission coil configurations it is always necessary to achieve an as high as possible degree of recognition reliability, i.e. to recognize any transponder entering the communication range of such a communication station and such a transmission coil configuration with a high degree of reliability, and to achieve an as high as possible degree of communication dependability, i.e. to communicate with such a transponder with a high degree of dependability. Even though in many cases a high degree of recognition reliability and communication dependability are achieved by the known communication station and the known transmission coil configuration, it has been found that for many applications and in many situations the recognition reliability and communication dependability that can be achieved are not satisfactory. The causes of such unsatisfactory functionality will be briefly elucidated hereinafter.
The known communication station and the known transmission coil configuration involve a continuously varying magnetic field distribution, because the coil currents through the active transmission coil and through the passive transmission coil exhibit a phase shift of 90xc2x0 relative to one another. The passive, second transmission coil circuit is tuned to a carrier signal frequency f1 which preferably amounts to 13.56 MHz. When a transponder enters the communication range, i.e. the magnetic field range between the first transmission coil and the second transmission coil, the transponder is supplied, via the carrier signal, with an adequate amount of energy required by the transponder in order to generate a response signal. The response signal is obtained by load modulation of the carrier signal, said load modulation causing amplitude modulation of the carrier signal so that there is formed an amplitude modulated carrier signal which has a sideband in the range of the sideband frequency f2 which preferably lies at approximately 14.0 MHz. When the transponder is situated nearer to the active, first transmission coil, the sideband in the range of the sideband frequency f2 is subject to a given resonance step-up which occurs due to the quality factor and the compensation circuit of the active, first transmission coil circuit. The level of the response signal received in the communication station is determined in that the transponder causes a small current variation in the active first transmission coil, which current variation causes a comparatively large voltage variation due to the resonance step-up at the sideband frequency f2; such a voltage variation can be suitably detected by receiving means of the communication station. However, if a transponder is situated nearer to the passive, second transmission coil, the same events take place as in the vicinity of the first transmission coil, but practically no resonance step-up at the sideband frequency f2 occurs for the passive, second transmission coil circuit. Consequently, a small current variation produced in the passive, second transmission coil causes only a small voltage variation, because the resonance step-up is practically absent or only very small via the inductive coupling between the passive, second transmission coil and the active, first transmission coil, such a small voltage variation causes only a comparatively small current variation in the active, first transmission coil, which itself becomes manifest as an only comparatively small voltage variation in the active, first transmission coil circuit. Consequently, when a transponder is situated nearer to the passive, second transmission coil circuit, the response signal applied to the receiving means of the communication station is substantially smaller than in the situation where a transponder is situated nearer to the active, first transmission coil circuit.
It is an object of the invention to eliminate the above described problems and to realize an improved communication station and an improved transmission coil configuration in which an equally good response signal is obtained in practically all operating situations, so that equally good and high recognition reliability and an equally good and high communication dependability.
In order to achieve the described object a communication station of the kind described in the first paragraph is according to the invention, characterized in that the first capacitor means and the second capacitor means both consist of a first capacitor stage and a second capacitor stage which are connected in series and connected to one another via a connection conductor, the first transmission coil circuit and the second transmission coil circuit both include a central conductor, a first end of which is connected to a central region of the relevant transmission coil and the second end of which is connected, via a third capacitor stage, to the connection conductor between the first capacitor stage and the second capacitor stage. In the second transmission coil circuit a resonance step-up is realized at the carrier signal frequency by means of the first capacitor stage and the second capacitor stage, and in the second transmission coil circuit a resonance step-up is realized at the sideband frequency by means of the third capacitor stage.
In order to achieve the described object a transmission coil configuration of the kind described the second paragraph is, according to the invention characterized in that the first capacitor means and the second capacitor means both consist of a first capacitor stage and a second capacitor stage which are connected in series and interconnected via a connection conductor. The first transmission coil circuit and the second transmission coil circuit both include a central conductor, a first end of which is connected to a central region of the relevant transmission coil and the second end of which is connected, via a third capacitor stage, to the connection conductor between the first capacitor stage and the second capacitor stage. In the second transmission coil circuit a resonance step-up is realized at the carrier signal frequency by means of the first capacitor stage and the second capacitor stage, and in the second transmission coil circuit a resonance step-up is realized at the sideband frequency by means of the third capacitor stage.
By taking the steps according to the invention it is achieved, while using very simple means and spending only little additional effort, that a resonance step-up is obtained at the sideband frequency in the passive, second transmission coil circuit; this offers the advantage that even a comparatively small current variation in the second transmission coil of the passive, second transmission coil circuit causes a comparatively large voltage variation due to the resonance step-up at the sideband frequency, which voltage variation is suitably transferred, by way of the inductive coupling, to the active, first transmission coil and hence can be suitably received and processed by means of receiving means connected to the active, first transmission coil circuit.
Each of the two central conductors in a communication station according to the invention and a transmission coil configuration according to the invention may be connected, by way of its first end, to the relevant transmission coil in a region other than the center thereof; such non-symmetry in the transmission coil circuits can be compensated for by means of unequal first and second capacitor stages. However, it has been found very advantageous to provide, a perfectly symmetrical construction of the transmission coil circuits while using particularly simple means; this has proven to be a major advantage in practice.
It has also been found in practice that it is also very advantageous to enable accurate tuning to the desired frequencies, for example the carrier signal frequency and the sideband frequency.
It has also been found to be very advantageous to provide amplitude matching of the coil current in the passive, second transmission coil with the coil current in the active, first transmission coil.
The foregoing aspects and further aspects of the invention are apparent from and will be elucidated with reference to the following embodiments.